The present invention relates to an H-shaped steel used as building structures. Particularly, the present invention relates to 590 MPa class heavy gauge H-shaped steel having a flange thickness of over 30 mm and a tensile strength of 590 to 740 MPa, and a method of producing the same.
Conventionally, box columns or welded H-shaped steel are frequently used as column materials of highrise or super highrise building structures. These box columns or welded H-shaped steel are formed by welding heavy gauge plates into box shape sections or H-shaped sections, respectively. With a column material required to have tensile strength at the level of 490 MPa or 520 MPa, an heavy gauge steel plates produced by controlled rolling and controlled cooling method, i.e., a so-called TMCP method, are welded. With a column material required to have tensile strength at the level of 590 MPa, a heavy gauge steel plates produced through two times of the quenching and tempering process are welded.
In contructing building, the reduction in construction cost and the shortening of construction time have recently strongly been required. Therefore, the use of rolled H-shaped steel as a substitute for box columns or welded H-shaped steel has been studied. In order to use rolled H-shaped steel, it is necessary to improve load carrying capacity. Specifically, it is required to use, as rolled H-shaped steel, high-strength heavy gauge H-shaped steel having a flange thickness of over 30 mm, and a quality level equivalent to or higher than thick steel plates of the box columns or welded H-shaped steel materials. There is also the tendency that from the viewpoint of earthquake proof, steel materials used for building structures including welded portions and weld heat-affected zones (referred to as xe2x80x9cHAZxe2x80x9d hereinafter) are required to have high toughness. This applies to high-strength heavy gauge H-shaped steel. In other words, high toughness is required not only in the rolling direction and in the direction of the flange width but also in the direction of the flange thickness. Similarly, HAZ is also required to have high toughness equivalent to a base material and a low susceptibility to cold cracking.
For example, Japanese Unexamined Patent Publication No. Hei-9-125,140 and U.S. Pat. No. 2,596,836 disclose that strength can be improved by using TMCP heavy gauge H-shaped steel produced by a structure controlling method for making a fine ferrite structure using an inclusion. However, heavy gauge H-shaped steel having strength improved to 590 MPa has a problem in that toughness in the direction of the flange thickness is insufficient. Also this heavy gauge steel has high Pcm which is an index for evaluating a weld cracking parameter, and thus has a problem in weldability.
On the other hand, in order to obtain 590 MPa class heavy gauge H-shaped steel, like a thick steel plate, two times of the quenching and tempering process may be applied. However, in order to form a martensite structure up to the center of the flange thickness, Pcm is inevitably increased. In addition, the hardness of HAZ is increased to cause the problem of deteriorating toughness. Furthermore, this process causes the problem of deteriorating dimensional precision due to heat treatment strain, and the problem of increasing cost, and thus has low practicability.
In other words, for the heavy gauge H-shaped steel provided in an as-rolled state, the composition and producing method, which can solve all of the above-described problems, are not yet established at present.
Japanese Unexamined Patent Publication Nos. Hei-8-85,846 and Hei-8-144,019, and U.S. Pat. No. 5,766,381 disclose that an appropriate amount of B is added to high-Mn extra-low-carbon steel to obtain a structure mainly composed of bainite, thereby obtaining a high-strength steel material having low dependency on a cooling rate. Particularly, these publications disclose that Pcm is significantly decreased by decreasing the carbon content to significantly improve weldability.
In accordance with recent research reports on the bainite structure and transformation behavior of low or extra low carbon steel (xe2x80x9cFinal Report of the Society of Bainite Researchxe2x80x9d, edited by the Society of Bainite Research, Basic Research Group, Iron and Steel Institute of Japan), typical micro structures of extra low carbon steel are classified into five types including Polygonal ferrite (referred to as xe2x80x9cxcex1Pxe2x80x9d hereinafter), Quasi-Polygonal ferrite (referred to as xe2x80x9cxcex1qxe2x80x9d hereinafter), Granular bainitic ferrite (referred to as xe2x80x9cxcex1Bxe2x80x9d hereinafter) bainitic ferrite (referred to as xe2x80x9cxcex1oBxe2x80x9d hereinafter), and Dislocated cubic martensite (referred to as xe2x80x9cxcex1xe2x80x2mxe2x80x9d hereinafter). The transformation temperature lowers in this order, and transformation is changed from diffusion type transformation to shear type transformation. It can be interpreted that the effect disclosed in the above-described Japanese Unexamined Patent Publication No. Hei-8-85846, etc. results from formation of xcex1B or xcex1oB. However, xcex1B and xcex1oB formed through the completion of bainite transformation inherit the state of xcex3 grains before transformation. In the hot deformation from a rectangular section to a H-shaped section, xcex3 grains which constitute the steel structure are crushed in the rolling direction and the width direction but less crushed in the direction of the flange thickness. Therefore, xcex1B or xcex1oB grains in the direction of the flange thickness are coarse as compared with grains in the rolling direction and in the direction of the flange width, adversely affecting toughness in the direction of the flange thickness. From the viewpoint of the mill ability, rolled heavy gauge H-shaped steel has the rolling restriction that large reduction cannot be applied, unlike a thick plate rolling mill. Since large reduction cannot be applied, xcex3 grains are possibly not sufficiently refined by recrystallization. This causes difficulties in refining the structure of the heavy gauge H-shaped steel by rolling, making demand for a method for advantageously removing deterioration in toughness in the direction of the flange thickness.
An object of the present invention is to advantageously solve the problems of production cost and strength, toughness and weldability of the product, i.e., to propose heavy gauge H-shaped steel having high toughness in the direction of the flange thickness, low Pcm and no hardening of HAZ, and a method of producing the same.
The inventors intensively studied the bainite transformation behavior of extra-low-carbon steel. As a result, it was found that in the bainite structure of extra-low carbon steel, more diffusive xcex1q grains finely dispersed in xcex1B to significantly improve toughness in the direction of the flange thickness while ensuring tensile strength at the 590 MPa level. Namely, it was found to be effective that strength is increased by decreasing the C amount against conventional common knowledge, and that Mn and Cu amounts are adjusted in appropriate ranges in order to finely disperse diffusive xcex1q grains. This resulted in the achievement of the heavy gauge H-shaped steel having excellent toughness in the direction of the flange thickness. Of course, Pcm is low because of extra-low-carbon steel, and thus excellent weldability is exhibited. It was also found that hardening of HAZ is not observed.
Namely, the construction of the gist of the present invention is as follows:
590 MPa class heavy gauge H-shaped steel has excellent as-rolled toughness in the direction of the flange thickness, and comprises 0.001 to 0.025 wt % of C, 0.6 wt % or less of Si, 0.4 to 1.6 wt % of Mn, 0.025 wt % or less of P, 0.010 wt % or less of S, 0.1 wt % or less of Al, 0.6 to 2.0 wt % of Cu, 0.25 to 2.0 wt % of Ni, 0.001 to 0.050 wt % of Ti, and 0.002 to 0.0030 wt % of B, wherein Mn/Cuxe2x89xa62.0 and 250xe2x89xa6117 Mn (wt %)+163 Cu (wt %)xe2x89xa6350 are satisfied. The 590 MPa class heavy gauge H-shaped steel further comprises one or two of 0.030 wt % or less of REM, and 0.0100 wt % or less of Ca, and/or at least one of 0.5 wt % or less of Cr, 0.5 wt % or less of Mo, 0.10 wt % or less of V, and 0.10 wt % or less of Nb.
A method of producing the 590 MPa class heavy gauge H-shaped steel having excellent toughness in the direction of the flange thickness comprises rolling steel slab having the above composition by a universal rolling mill, wherein after heating the steel slab at 1050 to 1350xc2x0 C., a portion of the H-shaped steel corresponding to a flange portion is rolled by using a rough universal rolling mill in the temperature range of 750 to 1100xc2x0 C. at a rolling reduction of 1 to 10% per pass, and a cumulative rolling reduction of 20% or more. The method of producing the 590 MPa class heavy gauge H-shaped steel further comprises cooling in the temperature range to 500xc2x0 C. at a cooling rate of 0.05xc2x0 C./s or more.